mgd,
To take leased civilian-owned aircraft into front-line military use is not that easy. There are complex regulatory, airworthiness and maintenance issues to overcome. For example you cannot just "slap-on" DAS boxes to an aircraft owned by someone else without their agreement. Their agreement would be heavily dependent on the airworthiness authorities (FAA/CAA) whose approval would be necessary to allow the subsequent return of the ac to revenue service with an airline.
Your idea is actually not that far away from what lies behind the concept of FSTA. The difficulties faced by the FSTA programme have principally been centred on such contractual wrangling rather than the customary procurement delays caused by technological and production issues.
FSTA will be the primary means of modernizing the equipment used to mount the Herrick airbridge and until that time the TriStar will have to soldier-on.